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| United States Patent |
5,140,060
|
|
Taguchi
, et al.
|
August 18, 1992
|
Electroluminescence device
Abstract
An electroluminescence device comprises a matrix polymer that is a polymer
obtained by polymerizing electron-accepting monomers having
electron-accepting groups. The electron-accepting groups are cyanoalkyl
groups and/or cyanoalkyl-oxyalkyl groups.
| Inventors:
|
Taguchi; Yoshihiro (Miyagi, JP);
Sato; Shunetu (Miyagi, JP)
|
| Assignee:
|
Alps Electric Co., Ltd. (Tokyo, JP)
|
| Appl. No.:
|
658924 |
| Filed:
|
February 21, 1991 |
Foreign Application Priority Data
| Current U.S. Class: |
524/555; 252/301.33; 252/301.35; 524/420 |
| Intern'l Class: |
C08K 005/16; C08K 003/30; C09K 011/08; C09K 011/02 |
| Field of Search: |
524/555,420
252/301.33,301.35
|
References Cited
U.S. Patent Documents
| 4708914 | Nov., 1987 | Kamijo | 428/690.
|
| Foreign Patent Documents |
| 2566791 | Jan., 1986 | FR | 252/301.
|
| 2305815 | Dec., 1990 | JP | 252/301.
|
Primary Examiner: Michl; Paul R.
Assistant Examiner: Szekely; Peter
Attorney, Agent or Firm: Shoup; Guy W., Kivlin; B. Noel
Claims
What is claimed is:
1. An electroluminescence device comprising:
a luminescent layer formed by dispersing fluorescent powder into a matrix
polymer, wherein said matrix polymer contains a polymer obtained by
polymerizing electron-accepting monomers having hydrophobic
electron-accepting groups.
2. The electroluminescence device as claimed in claim 1, wherein said
hydrophobic electron-accepting groups are cyanoalkyl groups.
3. The electroluminescence device as claimed in claim 1, wherein said
hydrophobic electron-accepting groups are cyanoalkyl-oxyalkyl groups.
4. The electroluminescence device as claimed in claim 1, wherein said
electron-accepting monomers having said hydrophobic electron-accepting
groups are acrylate monomers having cyanoalkyl groups.
5. The electroluminescence device as claimed in claim 1, wherein said
electron-accepting monomers having said hydrophobic electron-accepting
groups are acrylate monomers having cyanoalkyl-oxyalkyl groups.
Description
BACKGROUND OF THE INVENTION
1) Field of the Invention
The present invention relates to an organic dispersion-type
electroluminescence device (hereinafter called "EL device"), and
particularly to an EL device capable of providing a long life.
2) Description of the Related Art
In conventional organic dispersion-type EL devices, those formed by
dispersing fluorescent powder such as ZnS(Cu) into a polymer matrix have
been used as a luminescent layer. The polymer matrix, uses a high
dielectric constant polymer, such as cyanoethylated cellulose,
cyanoethylated poly(vinyl alcohol), etc.
However, the polymer such as the above 2-cyanoethylated poly(vinyl alcohol)
is produced by a method in which acrylonitrile is allowed to react with
poly(vinyl alcohol) in the form of a Michel-type addition reaction under
the presence of a basic catalyst and introducing 2-cyanoethyl groups into
hydroxyl groups of poly(vinyl alcohol). Therefore, the hydroxyl groups of
10% or so are left in its repeating structure without being subjected to
substitution, thereby increasing hygroscopicity due to the remaining of
the hydroxyl groups.
Thus, such a luminescent layer as referred to above also has high
hygroscopic capability, and the fluorescent powder in the luminescent
layer is decomposed by hygroscopic water content depending upon variations
with time, thus causing inconvenience such as a limitation on the
luminescent life of each EL device.
SUMMARY OF THE INVENTION
With the foregoing drawbacks in view, it is an object of the present
invention to provide an electroluminescence device having a luminescent
layer formed by dispersing fluorescent powder into a matrix polymer, the
electroluminescence device being characterized in that the matrix polymer
is a polymer obtained by polymerizing electron-accepting monomers having
electron-accepting groups.
The above-described polymer has side chains all of which are composed of
hydrophobic electron-accepting groups such as cyanoethyl groups. In
addition, the polymer has also high dielectric constant and low
hygroscopic capability. Thus, the hygroscopicity of the luminescent layer
of the type that the polymer or a blend polymer including the polymer is
used as a matrix is also improved, thereby making it possible to provide
an EL device which can make its luminescent life long.
As described above, since the EL device according to the present invention
is used with the polymer obtained by polymerizing the electron-accepting
monomers having the hydrophobic electron-accepting groups or with the
blend polymer including the polymer as the polymer matrix which forms the
luminescent layer of the EL device, the hygroscopic capability of the
polymer or the blend polymer becomes low and that of the luminescent layer
also becomes extremely low, thus bringing about an advantageous effect
that the luminescent life of the EL device becomes long. In addition, the
dielectric constant of the polymer can be rendered high, and a reduction
in drive voltages and an improvement in operational stability can be
carried out.
The above and other objects, features and advantages of the present
invention will become apparent from the following description and the
appended claims, taken in conjunction with the accompanying drawings in
which a preferred embodiment of the present invention is shown by way of
illustrative example.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a schematic view showing an EL device according to one embodiment
of the present invention; and
FIG. 2 is a diagram for describing infrared absorption spectrums of a
polymer 1 and cyanoethylated poly(vinyl alcohol).
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
A preferred embodiment of the present invention will hereinafter be
described with the accompanying drawings.
A description will first be made of a polymer which forms a luminescent
layer of an EL device according to the present invention.
The polymer employed in the present invention is a polymer obtained by
polymerizing electron-accepting monomers having hydrophobic
electron-accepting groups. As specific examples, may be mentioned an
acrylate monomer and a metacrylate monomer represented in accordance with
Formulas (1) and (2), etc.
CH.sub.2 =CHCOOX (1)
CH.sub.2 =C(CH.sub.3)COOX (2)
Functional groups X represented in the Formulas (1) and (2) are hydrophobic
electron-accepting groups, such as the following:
##STR1##
Since side chains of the above-described polymer are all represented in the
form of hydrophobic electron-accepting groups in the above-described
polymer, hydrophilic groups such as hydroxyl groups no longer exist in
molecules, thereby resulting in a reduction in water absorption
properties. Since the electron-accepting groups exist in the molecules in
large numbers, the dielectric constant of the polymer is high.
A description will now be made of the EL device used with the above polymer
which is regarded as a matrix of a luminescent layer or a dielectric
layer.
FIG. 1 shows the EL device according to one embodiment of the present
invention. A luminescent layer 4 formed by dispersing fluorescent powder
into a polymer matrix is interposed in a laminated form between a
transparent electrode 2 formed on one of both surfaces of a transparent
sheet 1 and an opposed electrode 3 composed of a metal foil such as
aluminum. The laminate thus formed is interposed between a pair of
protective sheets 5, 6 and sealed by tightly joining a peripheral edge
portion of the protective sheet 5 to that of the protective sheet 6 with a
heat seal used with a hot-melt adhesive, or the like. A lead-wire terminal
7 is electrically connected to an end of the opposed electrode 3 through a
conductive adhesive, whereas an unillustrated lead-wire terminal is
electrically connected to an end of the transparent electrode 2 through
the conductive adhesive. The luminescent layer 4 is emitted by applying
the voltage across the two electrodes 2 and 3 through the lead-wire
terminals from the outside. As the pair of protective sheets 5, 6,
materials having superb water-barrier properties may be used. They are
designed to prevent water from permeating the luminescent layer 4 provided
internally of the protective sheets 5, 6 to the utmost. As the protective
sheet 5 on the side of the transparent electrode 2, a resin film may be
used, for example, such as a transparent polytrifluorochloroethylene and a
polyethylene having superb water-barrier properties. On the other hand, as
the protective sheet 6 on the side of the opposed electrode 3, a metallic
composite film may be used having an extremely-high water-barrier
property, which is made by inserting a metal such as aluminum between the
resin films 8 and 8 such as a polyethylene terephthalate so as to be
formed in a laminated manner.
The luminescent layer 4 of the EL device is used with the above-described
polymer as a matrix. Specifically, the luminescent layer 4 is produced in
the following manner. Namely, fluorescent powder such as ZnS(Cu), ZnS(Mn),
ZnS(Al), etc. is added to the above polymer, and inorganic filler such as
fine silica powder (Aerosil) is added to the same as needed. Further, they
are mixed together in an organic solvent such as .gamma.-butyrolactone so
as to obtain a dispersed liquid composition. The thus-obtained composition
is coated or applied on the transparent electrode 2 by the screen printing
process or the like, followed by drying, thereby obtaining the luminescent
layer 4. The luminescent layer 4 is normally on the order of 10 .mu.m to
100 .mu.m in thickness. The fluorescent powder in the luminescent layer 4
is contained in an amount of from about 70 to 90 wt. %, and the polymer is
contained in an amount of from about 10 to 30 wt %.
In the EL device described above, the polymer matrix, which forms the
luminescent layer 4 of the EL device, is a polymer having the
extremely-low hygroscopic capability as described above. Therefore, the
hygroscopic capability of the luminescent layer 4 becomes extremely low,
and hence decomposition of the fluorescent powder by water is restricted,
thereby making it possible to make the luminescent life of the EL device
significantly long.
Since its dielectric constant is also high as described above, the drive
voltage can be reduced and the operation of the EL device can be kept
stable.
As a modification of the EL device, there is provided one of the type in
which the luminescent layer 4 is composed of two layers, i.e., a layer of
a fluorescent substance and a layer of a dielectric. In this case, the
fluorescent layer is formed as follows. Namely, the fluorescent powder and
the polymer are mixed together in the organic solvent to obtain the
dispersed liquid composition, and the so-obtained composition is applied
on the transparent electrode 2 in the same manner as described above,
followed by drying, thereby forming the layer. On the other hand, the
dielectric layer is formed in the following manner. Namely, one obtained
by dissolving the polymer in the organic solvent or the liquid composition
obtained by adding inorganic dielectric powder such as barium titanate
powder to the polymer as needed is applied on the fluorescent layer and
then dried, thereby forming the dielectric layer. An increase in the
stable operation of the EL device constructed in this way is further
secured.
Incidentally, the application of the above polymer to both of the
fluorescent layer and the dielectric layer or to either one of them can be
selected arbitrarily.
EXAMPLES
Specific examples will hereinafter be described.
(A) Polymerization of Polymer 1
An air inside a two-way type flask provided with a jacket-type cooler is
replaced with nitrogen. Then, 0.4 mol of 2-cyanoethylacrylate, 20 ml. of
acetone and 2-2'-azobisisobutyronitrile (2 mol % with respect to the total
weight of monomers), all of which are purified, are introduced into the
inside thereof, followed by polymerization at 60.degree. C. for 24 Hrs.
Thereafter, the polymer is allowed to be reprecipitated in methanol so as
to obtain an intended polycyanoethylacrylate.
(B) Polymerization of Polymer 2
Using CH.sub.2 =CHCOOCH.sub.2 CH.sub.2 CH.sub.2 CN as an alternative to the
use of the 2-cyanoethylacrylate, the polymer is obtained in the same
manner as the polymerization process of the above polymer 1.
(C) Polymerization of Polymer 3
Using cyanoethylmetacrylate as an alternative to the cyanoethylacrylate,
polycyanoethylmetacrylate is obtained in the same manner as the
polymerization process of the above polymer 1.
The dielectric constant (25.degree. C., 1 KHz), the hygroscopicity and the
amount of residual water of each of the polymers 1 to 3 thus obtained have
been examined.
The evaluation of the hygroscopicity is performed in the following manner.
An acetone solution of the polymer is applied on a suitable carrier by the
cast film process so as to produce a film of 0.1 mm in thickness. The
thus-produced film is dried in vacuo at 100.degree. C. for 24 Hrs. After
drying, the amount of water in the film is measured by the Karl Fischer
method, thereby determining the result as the amount of residual water. In
the polymer, the film is left over 24 Hrs under conditions that
temperature is 40.degree. C. and RH is 90-95%, after which the amount of
water included in the film is measured. The term vol % with respect to the
weight of the film is set as a value indicative of the hygroscopicity.
The result of the above examination will be shown in Table 1. Values
obtained by examining the conventional cyanoethylated poly(vinyl alcohol)
in the same manner are described in Table 1 as compared values together
with values of the above result. Incidentally, the percentage of
cyanoethylation for the cyanoethylated poly(vinyl alcohol) was about 90%.
FIG. 2 is a diagram for describing infrared absorption spectrums of both
the polymer 1 and the cyanoethylated poly(vinyl alcohol). It is understood
from the drawing that the polymer 1 has a reduced absorption peak in the
neighborhood of 3300 cm.sup.-1, which is caused by the water or a hydroxyl
group (OH), as compared with the cyanoethylated poly(vinyl alcohol), thus
resulting in low water-absorption properties.
TABLE 1
______________________________________
Amount of
Dielectric
residual water
Hygroscopicity
constant
(wt %) (wt %)
______________________________________
Polymer 1
19 0.047-0.078 2.3-2.4
Polymer 2
21 0.043-0.08 2.5-2.9
Polymer 3
10.5 0.025-0.042 1.4-1.9
Cyano- 14 0.1-0.21 5.5-9.2
ethylated
poly(vinyl
alcohol)
______________________________________
A description will next be made of Examples of EL devices used with the
polymers described above.
EXAMPLE 1
The polymer 1 is used to form a luminescent layer, thereby producing an EL
device from the so-formed luminescent layer. ZnS(Cu).sub.2 is used as
fluorescent powder. Then, the polymer 1 (40 g) is dissolved in 60 g of
.gamma.-butyrolactone. Thereafter, 100 g of a dispersed solution obtained
by this dissolution and 41 g of a dispersed solution obtained by
dispersing 1 g of an aerosil into 40 g of the .gamma.-butyrolactone are
mixed together and dispersed in a uniform manner, thereby obtaining a
liquid composition. The liquid composition is applied on a transparent
electrode of a transparent sheet, followed by drying, thereby forming a
fluorescent layer having a thickness of about 30 .mu.m.
Then, the polymer 1 (40 g) is dissolved in 60 g of the
.gamma.-butyrolactone. 100 g of a dispersed solution obtained by this
dissolution and a dispersed solution obtained by dispersing 1 g of the
aerosil into 40 g of the .gamma.-butyrolactone are mixed together and
dispersed in a uniform manner, to thereby obtain a liquid composition. The
thus-obtained liquid composition is applied on the fluorescent layer,
followed by drying, thereby forming a dielectric layer having a thickness
of about 50 .mu.m.
Further, an opposed electrode made of an aluminum foil is formed on the
so-formed dielectric layer so as to obtain laminates. The laminates are
covered and sealed by a pair of protective sheets of a polyethylene film
and a polyethylene terephthalate/aluminum composite film so as to produce
an EL device 1.
EXAMPLE 2
An EL device 2 is produced in the same manner as in Example 1 except for
the use of the polymer 2 employed instead of the polymer 1.
EXAMPLE 3
An EL device 3 is fabricated in the same manner as in Example 1 except for
the use of the polymer 3 employed instead of the polymer 1.
EXAMPLE 4
An EL device 4 is produced by using a blend polymer composed of both 65
parts by weight of cyanoethylated pluran (percentage of cyanoethylation is
about 90%) and the polymer 1 (35 parts by weight) instead of the polymer 1
in the same manner as in Example 1. The dielectric constant of the blend
polymer was a range from 17.2 to 18, the amount of residual water was a
range from 0.55 to 0.95 wt %, and the water-absorption property thereof
was a range from 2.7 to 4.2 wt %.
COMPARATIVE EXAMPLE
An EL device is fabricated in the same manner as in Example 1 with the
exception of the use of the blend polymer composed of 65 parts by weight
of the conventional cyanoethylated cellulose (percentage of
cyanoethylation is about 90%) and 35 parts by weight of cyanoethylated
poly(vinyl alcohol) (percentage of cyanoethylation is about 90%), as an
object to be compared. Incidentally, the dielectric constant of the blend
polymer was a range of 14.1 to 16.5, the amount of the residual water was
a range of 0.55 to 0.95 wt %, and the water-absorption property was a
range of 5.2 to 5.5 wt %.
The luminescent life of each of the five kinds of EL devices produced in
the above-described manner was measured by applying a 100 volts, 400 Hz
a.c. power supply across electrodes and continuously emitting them under
the conditions that a testing temperature is 20.degree. C. and RH is 60%.
The luminescent life is represented by the half-life period required for
the initial luminance to be reduced to one-half its initial value.
Table 2 shows the initial luminance and the luminescent life.
TABLE 2
______________________________________
Luminescent Life
Initial Luminance
(Hours) (cd/m.sup.2)
______________________________________
Example 1 3600 49.2
(EL device 1)
Example 2 3300 55.4
(EL device 2)
Example 3 4200 41.5
(EL device 3)
Example 4 3500 53.2
(EL device 4)
Comparative 2700 49.5
Example
(Conventional
Example)
______________________________________
It is clearly understood from Table 2 that the EL device of the present
invention using the polymer can extend the luminous life-time thereof
significantly because the hygroscopic capability of the polymer is low. In
addition, it is clear that the blend polymer composed of the above polymer
and a crystalline polymer such as cyanoethylated cellulose, cyanoethylated
pluran, etc. also exhibit a long-lived effect.
Having now fully described the invention, it will be apparent to those
skilled in the art that many changes and modifications can be made without
departing from the spirit or scope of the invention as set forth herein.
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